Their classes as you say are clear but this doesn't do them justice. They are uniquely clear. There is no other place -- iTunes U, Masters courses, etc -- that reaches their level of clarity. And, as you point out, for the toughest subjects.
If I don't take Udemy, I can generally read the documentation or read through umpteen online tutorials and figure out how to get an app up in Rails/Django/React etc.
If I don't take the Udacity couse on (for instance) AI for trading that information would simply not be available short of a specialized course at one of the top universitys and even then it might not be of the same quality. I studied Machine Learning for my graduate degree and the quality is the same as the best professors there (as it should be! They took the clearest lectures and perfected it before they recorded and edited it).
I don't usually get fired up about online course providers but they have earned it.